The presence of high levels of nitrates in water is known to cause harmful physiological effects in humans. Consequently, potable water supplies are typically treated in an ion exchange process to at least reduce the nitrate levels. During some point in an ion exchange process, it is necessary to regenerate the anion exchange resins by washing with a regenerant. This is typically achieved by using a single pass of concentrated brine, i.e., water which is nearly saturated with salts, through the resin columns of the ion exchange process. Sodium chloride (NaCl) brine is most often utilized because of its low cost.
Unfortunately, the advantages of using NaCl brine are offset by the high cost of disposal of the resulting waste brine which contains nitrate and chloride ions. If a method could be found to remove the nitrate ion relative to the chloride ion, then the waste brine could be reused as a regenerant.
One of the most widely used methods of removing nitrate ion from waste brine, biological denitrification, suffers from several drawbacks associated with the use of living organisms. These drawbacks include undesirable dilutions to avoid high ionic strength problems with microorganisms, difficulties in maintaining a viable culture of bacteria, high cost of chemicals to maintain the bacterial culture and unpredictable reaction rates. The method also utilizes relatively large equipment. In addition, the use of bacterial cultures can result in contamination of drinking water.
Another method of removing nitrate ion from waste brine is biological recycling. Like biological denitrification, this method also suffers from the drawbacks associated with maintaining living organisms. These drawbacks include high nutrient costs to keep the bacterial culture alive, possible contamination to drinking water, and slow and/or unpredictable reaction rates. In addition, because the waste brine must be diluted to allow the microorganisms to denitrify it, the process requires the additional time-consuming step of reconcentrating the brine to regenerate the exchangers.
Other methods of removing nitrate ion from waste brine include the selective ion exchange method disclosed in U.S. Pat. No. 4,479,877 to Guter and an oxidation-reduction scheme disclosed in U.S. Pat. No. 5,069,800 to Murphy. The Guter process tends to be relatively expensive because of the cost of ion exchange resins which lose capacity over time due to oxidation, as well as the high cost of regenerant solutions and their associated disposal costs. Similarly, there are considerable expenses associated with the reducing agent used in the Murphy process and the undesirable reaction products left in the effluent require additional post-treatment steps.